1. Field of the Invention
This invention relates generally to techniques for improving the performance of polyphase alternating current (AC) machines by injection of harmonic frequencies into the excitation current. More particularly, this invention relates to improved techniques for generating harmonics of a fundamental frequency source and providing an adjustable phase offset between the harmonic and the fundamental frequencies.
2. Description of Related Art
Polyphase machines of both the induction and synchronous type are typically operated from a single frequency source and have armature windings distributed in slots in such a way as to approximate a constant-amplitude, sinusoidally-distributed flux wave traveling around the rotor air-gap at synchronous speed. Ideally, the flux wave so generated produces a steady electromagnetic torque having a maximum value dependent upon the air-gap flux per pole and the current-carrying capacity of the armature winding. Because the maximum flux density of the air-gap flux wave is limited by the saturation flux density of the material (typically magnetic steel) existing in the flux path, the air-gap flux per pole produced by the flux wave is also correspondingly limited.
Using the flux-per-pole of a sinusoidal flux wave as a reference and maintaining a constant peak flux density, the flux per pole can be increased by the addition of odd harmonics of appropriate amplitude to the fundamental wave. The increased flux reaches a maximum value as the combination of the fundamental frequency and odd harmonics added thereto approaches a square wave. The combination, for instance, of a third harmonic wave and the fundamental wave can produce up to a 23% increase in the flux per pole relative to the sinusoidal reference, provided the amplitude of the waves are appropriately matched. The increase in air-gap flux per pole resulting from the added space harmonics can be used to increase the power output of a polyphase machine without exceeding thermal dissipation limits. In particular, a significant increase in the output power and performance of an induction motor can be achieved with the addition of third-harmonic flux. The application of third-harmonic excitation is also advantageous in that core losses and excitation currents can be reduced (when the magnetic path is saturated) in polyphase transformers and rotating machines; the reduced core losses directly contribute to increased efficiency and rating.
An exemplary method for improving the performance of polyphase machines by using the combination of a fundamental frequency and an odd harmonic thereof is described in copending U.S. patent application Ser. No. 888,818, filed Jul. 22, 1986, entitled "Method And Apparatus For Improving Performance Of AC Machines", which is also owned by the assignee of the present invention. As described therein, the excitation of odd harmonics serves to improve performance of an AC machine in two ways: (a) the flux distribution introduced by the added harmonic excitation produces increased fundamental distribution of flux densities along the magnetic path so as to improve the magnetic loading of the material, and (b) the added harmonic flux distribution yields increased output torque if the machine is provided with conductors or coils responsive to the armature harmonic frequencies or if the rotor poles produce permeance waves responsive to the harmonic frequencies. This concept of flux modulation by third-harmonic current injection is becoming increasingly common as a means for enhancing the performance of electric power apparatus.
A critical aspect of implementing harmonic injection schemes is of course the provision of a reliable harmonic frequency source. It is also important that such a harmonic generator be capable of providing an adjustable phase angle for the harmonic voltage with reference to the fundamental voltage in order that an optimum flux pattern be obtained and the harmonic phase angle be locked to the desired phase angle. The generation of odd harmonic frequencies, particularly the third harmonic, is, however, not restricted to applications employing the flux modulation technique. Generators of this type are also essential in a variety of other applications such as those using harmonic distortion for increasing the output voltage of polyphase PWM inverters, multiplex converters, and AC-to-DC converters. For instance, in AC/DC controlled converter applications, the rectangular supply current can be modified by addition of a third harmonic current of appropriate amplitude and phase in order to reduce undesirable harmonic content in the supply current.
A variety of approaches are presently used for generating harmonic components along with the fundamental wave component. Different methods, ranging from the use of separate coils for fundamental excitation and for each odd harmonic excitation to the use of a common set of delta-connected windings actuated through a multiphase inverter, are employed. U.S. Pat. No. 3,970,914 to Salzman et al.("Salzman"), for instance, describes the production of a third-harmonic component together with a fundamental component through the use of inverters. U.S. Pat. No. 4,264,854 to Hawtree ("Hawtree") discloses the use of a plurality of similar digital counters in order to produce multiphase signals and to regulate phase displacement between armature windings. Another example is the technique proposed by Power Liou ("Theoretical And Experimental Study Of Polyphase Induction Motors With Added Third Harmonic Excitation", thesis for M.S.E.E., the University of Texas at Austin, December 1985; "Liou") which is based on generating a single-phase third-harmonic signal by using a triac circuit. Other schemes include those based on phase-shifting a line-frequency signal to produce third-harmonic multiphase gate timing signals which are fed to the solid-state switches of an inverter circuit, using synchronous generators with high-power converters and oscillator-amplifier systems with low-power converters, and using the DC ripples of a six-pulse converter as a basis for generating desired harmonics. It is also possible to use a phase-locked oscillation control scheme together with an appropriate logic system to control the phase angle and frequency relationship between the fundamental frequency input and the harmonic frequency output.
A common problem with conventional third-harmonic generation schemes is the typically high cost and circuit complexity of the harmonic generator required to attain desired performance goals. In addition, conventional harmonic generators are incapable of generating independent third-harmonic gating signals which have the harmonic phase angle conveniently adjustable relative to the fundamental wave. Another problem is that the third-harmonic component is typically produced as a distorted version of the fundamental sine wave (see, for e.g., the Salzman disclosure). Existing digital techniques also suffer from the problem of error accumulation over the different harmonic cycles (see, for e.g., the Hawtree disclosure) and generally require complicated harmonic generation and filtering circuits (see, for e.g., the Liou disclosure). Closed-loop schemes are particularly costly because of the complex circuitry involved in using a closed-loop adjustment to minimize the errors between the desired harmonics and the actual output harmonics.
There accordingly exists a need for uncomplicated, low-cost, and high-performance generation of odd-harmonic gating signals having conveniently adjustable phase angles with respect to the fundamental frequency wave.